Electrolytic production of high-purity gallium



R. I. STEARNS 3,423,301

PURITY GALLIUM Jan. 21, 1969 ELECTROLYTIC PRODUCTION OF HIGH Filed Nov. 2, 1964 POWER SUPPLY H COLD WATER COLD WATER INVENTORI ROBERT l. STEARNS YM -My,

ATTORNEY United States Patent 3,423,301 ELECTROLYTIC PRODUCTlON 0F HIGH-PURITY GALLIUM Robert I. Stearns, St. Louis, Mo., assignor to Monsanto Company, St. Louis, Mo., a corporation of Delaware (Jontinuatiou-in-part of application Ser. No. 245,189, Dec. 17, 1962. This application Nov. 2, 1964, Ser. No. 410,344 U.S. Cl. 204-105 Int. Cl. C22d 1/10 This invention is a continuation-in-part of my copending US. application Ser. No. 245,199, filed Dec. 17, 1962 and now abandoned.

This invention relates to an improved method for the preparation of gallium. More particularly this invention relates to an improved process for the refining and production of gallium of very high purity, which process comprises the electrodeposition of ultra-pure gallium from an acidic solution. This invention further relates to an apparatus useful in carrying out the process described herein.

Previous electrodepositions of gallium have been carried out in alkaline mediums utilizing electrolytes such as sodium gallate. Such a process requires the use of an alkaline material such as sodium hydroxide, which use results in contamination of the product with sodium as well as other substances present as impurities in sodium hydroxide. Elimination of these contaminants would result in a superior product for the preparation of semiconductor materials, one of the principal uses for gallium today. Theoretically, electrolysis in an acid medium is capable of producing gallium free from metal impurities, but previous attempts to product the metal by such a method have failed. Because of the inherent advantages of an acidic electrolysis of a gallium solution, much time and effort have been expended by researchers in an attempt to devise a workable process. And yet, according to Hampels Rare Metals Handbook (1961), there is no record of success in attempting to electrolyze acid solutions of gallium as a means of production. The principal difficulties to the successful electrodeposition of gallium in an acidic medium have been poor current efficiencies and formation of solid gallium oxychloride, GaOCl, which prevents the formation of metallic gallium.

It is an object of this invention to provide an improved process for the production of high-purity gallium. It is a further object of this invention to provide an improved process for the continuous electrolysis of an acid solution of gallium with the resultant deposition of metallic gallium of very high purity. It is yet another object of this invention to provide a novel apparatus for use in the continuous electrodeposition of gallium metal. Additional objects and advantages will become apparent from the discussion following.

The phrase continuous electrolysis as used above and elsewhere in the disclosure and claims, shall be construed to mean an electrolysis capable of operation for an extended period of time free from the difficulties previously encountered in electrolyses of acidic solutions of gallium salts.

The continuous electrolysis of an aqueous acidic solution of gallium ions resulting in the deposition of metallic gallium is achieved by adding to the above-mentioned solution during the electrolysis sufficient quantities of gallium ions and halide anions to maintain the mole ratio of [anion]/[Ga+ between 2.5 to 1 and 3.5 to 1 during the operation of the process. Gallium halides have a stoichiometric [anion]/[Ga+ ratio of 3 to 1. The requirements of this invention demand that this ratio be maintained as closely as possible to its original value throughout the electrolysis when a gallium halide is used 8 Claims 3,423,301 Patented Jan. 21, 1969 as the electrolyte. Preferred is a range of stoichiometric ratios from about 2.9 to 1 up to about 3.1 to 1. Even this range will not provide as good results, however, as a ratio more closely approximating the stoichiometric ratio of 3.0 to 1. When the ratio drops below about 2.5 to 1 or rises above about 3.5 to l, the difficulties encountered in the prior art manifest themselves to such an extent that the objects of this invention are no longer achieved.

Any gallium halide can be used in the practice of the present invention. Likewise, any of the halogen acids can be used to provide a source of available anions for the electrolysis. Preferred reactants are gallium chloride, GaCl and gallium bromide, GaBr in combination with either hydrochloric or hydrobromic acids. The other gallium halides, gallium fluoride and gallium iodide, can nevertheless be used herein, either in combination with the corresponding halogen acid, hydrofluoric or hydro iodic acid, or with one or more of the other halogen acids.

The requirement of controlling the concentration levels of the electrolyte and acid was incorporated into the process after observing the electrolysis of an aqueous solution of GaCl using a molten gallium cathode with a tungsten contact and a high density graphite anode. Phenomena observed were as follows:

(a) Upon dissolving solid GaCl in water, an acidic solution results.

(b) Electrolysis at first proceeds with little or no hydrogen gas produced at the cathode.

(c) As the electrolysis proceeds further, the amount of hydrogen gas produced at the cathode becomes increasingly greater.

((1) Finally the electrolyte becomes clouded by a precipitate of GaOCl and the current efi'iciency with respect to the deposition of gallium metal approaches zero. The pH of the solution at this point is higher than that of the freshly prepared solution.

(e) If hydrochloric acid is added in increasing amounts to a 'GaCl solution, the current efiiciency with respect to the gallium deposition decreases, finally approaching zero.

Observation (a) is readily explained by noting that hydrolysis of a salt of a strong acid and weak base (HCl and Ga(OH) herein) results in an acidic solution. The pH of a 2M GaCl solution is approximately 1.0

In order to explain (-b), it is necessary to apply the Nernst Equation. Thus for the system Ga Ga+ +3e, where the Ga concentration is 2M and E0=0.52:

In the same solution, the pH is about 1. Therefore for /2H H++e The discharge potential of hydrogen in this system is calculated by adding the hydrogen overvoltage 0.44 at a molten gallium cathode.

The two reduction potentials 0.51 and 0.50 indicate that in a freshly prepared aqueous solution of GaCl the reductions take place with about equal ease and roughly in proportion to their relative concentrations which greatly favors the Get at the outset. Because of similar reasons the current efficiency at the anode for the oxidation of Cl to chlorine gas is higher than at the cathode for the reduction of Ga to gallium metal. This results in the phenomena indicated in (c) and (d). Several things are occurring to produce these phenomena. The gallium concentration is being decreased by the formation of gallium metal. The pH is increased by the formation of hydrogen gas which finally results in the precipitation of gallium oxychloride, GaOCl. Thus the gallium ion concentration is finally reduced effectively to zero and further electrolysis produces only hydrogen at the cathode. The addition of hydrochloric acid, 2 above, results eventually in the reversal of the relative concentrations of the gallium and hydrogen ions. This situation results in the formation of only hydrogen gas.

In view of the preceding discussion, many simple variations in the disclosed process should become obvious to those skilled in the art. For instance, modification of the method described in the example to produce greater or lesser amounts of gallium is not necessarily achieved by adjusting all the reagents by one factor. However, such a theoretical explanation as given here will suggest many modifications to those skilled in the art, thus enabling them to devise a method for the production of varying amounts of gallium. Other modifications may also become apparent, such as the production of other elements whose characteristics resemble those of gallium. This is the purpose of the theoretical discussionto provide a basis for such modification. It should not be construed as an integral part of my invention so as to constitute a limitation thereof.

These observations, together with the explanations for their occurrence, indicate that, in order to conduct a continuous electrolytic reduction of GaCl to metallic gallium, it is necessary first to replenish the Ga+ concentration as gallium metal is formed, and secondly to control the acidity so that GaOCl precipitate does not form and so that the formation of hydrogen gas does not become the predominate reduction at the cathode. The first requirement is accomplished by addition of a concentrated stock solution of GaCl and secondly by removal by distillation or evaporation of excess water in the system. The heat required for this water removal is supplied by the electrical energy input necessary for the electrolysis or by external heating. The second requirement is satisfied by the addition of concentrated hydrochloric acid in amounts necessary to restore the [Cl]/ [Ga+ ratio to 3. Excess water is also removed. In a solution with the 621+ and Cl concentrations controlled in such a manner, the gallium reduction proceeds at high current efficiencies exceeding 95% for short periods. A method utilizing this discovery proceeds as follows: a quantity of pure gallium metal is used as the cathode; a pure graphite rod is used as the anode. The electrolyte is composed of a gallium salt solution of known concentration. The electrolysis is started and the current, the distance of electrodes from each other, and the periodic additions of measured quantities of concentrated electrolyte and acid are so interdependently adjusted as to maintain approximately the original pH and [anion]/[cation] ratio of the solution.

Many variations in equipment and operating conditions are conceivable but certain features are desirable.

(1) There should be continuous agitation of the solution. This is best provided by an air-lift pump operated by nitrogen.

(2) The escaping quantities of hydrogen and chloride gas should be diluted by an inert gas such as nitrogen to reduce the danger of explosion.

(3) The graphite anode and molten gallium cathode areas should be such as to have current densities of 0.3 to 1.0 amps. per crn. at each electrode, and they should be horizontally mounted parallel to each other at a dis tance that passes the required current of 50-100 amps. at a potential of 12-16 volts. It is advisable to perforate and channel the anode to facilitate the escape of chlorine gas.

(4) A cooling arrangement to keep the operating temperature within the range of 7090 C. is desirable.

(5) The only materials coming in contact with the gallium chloride solution should be Pyrex, quartz, or Teflon. Other materials are badly attacked and offer sources of contamination.

(6) Since no greases or lubricants should be used on tapered joints or the ball and socket connections, tapered joints should be sealed with some material such at Teflon sleeves, and the ball and socket joints should be wrapped with some material such as Teflon pressure-sensitive tape and then clamped. Several layers of Teflon tape should also be applied to the graphite anode and the tungsten contact rod just above the pressed Teflon fittings to prevent the entry of dust and to hold the two items in position.

One such apparatus which provides these desirable features is shown herein. In general, this apparatus comprises an enclosed electrolysis vessel containing a pool of metallic gallium which is provided with a source of electricity to cause the pool to function as a cathode. Also contained within the vessel is an anode, preferably of ultra-pure graphite. Attached to the vessel is a reflux column which can be converted to discharge condensate instead of returning it to the electrolysis vessel. This conversion from refiux to discharge is effected when the level of electrolyte reaches the point where the ratio will be changed significantly. This change in electrolyte level can be observed visually or through some automatic sensing device and then adjusted manually or automatically.

The following examples and accompanying drawing of the apparatus will further explain the invention. The apparatus and the quantities mentioned in the example were designed for the production of 400 grams of gallium in a six hour period. Simple modifications are apparent to adapt this procedure for production of greater or lesser amounts.

It should also be obvious to one skilled in the art of commercial production that such an operation as will be subsequently described is capable of completely or nearly completely automatic control and adjustment. For instance, using the equipment and procedure outlined below, it is advisable to maintain a visual check on the operation and make manual adjustments as necessary to avoid precipitation of GaOCl or too rapid evolution of hydrogen gas. It is also advisable to withdraw a sample of electrolyte at periodic intervals of six to ten hours for analysis. This allows the electrolyte to be adjusted for maximum gallium production. An apparatus to provide for automatic sample withdrawal and analysis with subsequent adjustment of the electrolyte is feasible. Other similar modifications could likewise be introduced. It is within the scope of this invention to apply its use to such an automatic device as Well as to the apparatus described below and shown in FIGURE 1.

Example 1 High purity gallium chloride was prepared from crude gallium metal by direct chlorination of the metal followed by two simple distillations and one fractional distillation of the chloride. The gallium chloride so prepared was used as the electrolyte in this final purification step. A 2 M GaCl aqueous solution was prepared and introduced into the electrolysis vessel 9. About 400 grams of molten gallium was poured down the tungsten contact support tube 11 until a small quantity spilled over into the settling chamber 8. The solution above the gallium in the support tube was removed by suction to prevent attack on the tungsten by hydrochloric acid. This tube was then rinsed a few times with triple-distilled water. After the last rinse was removed by suction, the tungsten contact rod 19 was put into place. During this operation the gallium pool 10 which was subsequently used as the cathode was warmed slightly to prevent its solidifying. This gallium cathode pool was connected to the power supply 27 by means of a tungsten rod 19 of the highest purity and a brass clamp 21. The anode 18 for this electrolysis consisted of a high purity graphite rod made by United Carbon Products, Grade UF-4-S. This anode was drilled and slotted along the portion which lies in a plane parallel to the cathode pool. This was done to facilitate the escape of hydrogen and chlorine gases. The anode was held in place by a support collar 12 with. a pressed Teflon fitting and was connected to the power supply 27 by means of a brass clamp 20.

The nitrogen lift pump 7 was started by introducing nitrogen gas at the filter tube 23. This flow was then adjusted to deliver approximately 900 or 1000 ml./min. as read on the flow meter 22. The nitrogen purge was started through the top of the graphite filter 6 and reservoir supports 5 at a slow rate of perhaps 30 or 40 ml./minute. It was not necessary to meter this flow. A vent 17 at the top of the graphite filter 6 was provided to relieve any excess pressure not required to carry the concentrated electrolyte and acid to the electrolysis cell 9. With the nitrogen pump operating, the electrolyte level in the center chamber of the graphite filter 6 and reservoir supports 5 was marked. The electrolyte level in the arm of the graphite filter and reservoir supports through which the concentrated HCl is introduced was marked with a worm type rubber tubing clamp 24. The clamp was placed along the exterior surface of reservoir support 5 just above the level of the electrolyte. The clamp was then connected by a short piece of bare silver wire to a Thermocap relay 25, manufactured by Niagara Electron Laboratories. This relay is a power controlling instrument actuated by minute changes in electrical capacity. The clamp was positioned and the relay adjusted so that any rise in the level of the electrolyte would change the capacitance between the plates of the clamp 24. This change in capacitance would close a circuit within the relay 25, thereby actuating the solenoid 26, which in turn would direct the flow of distillate to a discharge drain instead of to the reflux drain. The direction of distillate flow is varied by changing the position of the flow-return funnel within the liquid divider 13. This result can be accomplished by means of an electromagnet within the liquid divider 13. When the electromagnet is operating, the flow-return funnel can be pulled into a position so that the flow of distillate is directed into a discharge drain instead of to the reflux drain. When the liquid level of the apparatus returns to a proper level, the electrolyte will fall in the reservoir suport, thereby restoring the original capacitance to the clamp 24, opening the relay 25, deactivating the solenoid 26, releasing the electromagnet within the liquid divider 13, and returning the flow-return funnel to the refluxing drain. During the operation of the system, water vapor is carried along with the nitrogen, hydrogen and other gases formed from the oxidation of anions in solution, up into the cold water condensers 14 and 15. The gases continue on through vent 16 but the water is condensed and returned to the liquid divider Where it is refluxed or discarded, depending upon the quantity of electrolyte present in the electrolysis cell. This mechanism is helpful in a continuous process such as this where quantities of additional solutions are added periodically throughout the electrolysis.

The cold water was started through the condensers. Then a timer to measure total clasped time, the HCl addition timer 3, the GaCl addition timer 4, and the power supply 27 with an automatic current monitor adjusted to deliver 75 amperes, were turned on simultaneously. The concentrated H'Cl reservoir buret 1 delivered a total of 5.7 ml. at 6.5 minute intervals, and the M GaCl reservoir buret 2 delivered 27.0 ml. at 226 minute intervals. These additions were arrived at empirically and do not necessarily represent the best possible quantities of material or rates of addition. The electrolysis was continued for 450 minutes using 75 amps. at a potential of 12-15 volts. The voltage was controlled by adjusting the distance between the gallium cathode pool 10 and graphite anode 18. As the gallium pool 10 increased in size, it spilled over into the tube 28 and was carried to the settling bottle 8. At the end of this time of 450 minutes, the electrolysis was stopped. After removing the tungsten contact rod 19 and stoppering the support tube 11, the gallium was removed both from the settling chamber 8 and the cathode pool area 10 by applying suction at these points.

Example 2 according to the directions given for each case.

[Ga+ 2 M [Ga+ 2 M [Ga+ 2 M [Anion]/[Ga+ ]-3 Case A Case B Case 0. [Anion]/[Ga 3 Case D Case E Case F. Anlon]/[Ga+ 3 Case G Case H Case I.

Case A: No adjustments necessary. Case B: Reduce the gallium concentration to approximately 2 M by electrolyzing at amps. for the time calculated by the equation tmin ([Ga+ ]2.0) (V96500) (0.06972) (75) (60) (23.24) (0.8)

= ([Ga fl-ZO) V-volume of the electrolyte (approx. 1500 ml.). For this period add concentrated halogen acid as described above in the example electrolysis.

Case C: Increase the gallium concentartion by adding the volume of 10 M gallium halide calculated by the equation Case D: Increase the halide concentration by adding the volume of concentrated halogen acid as calculated by the equation Case E: First add concentrated halogen acid as calculated in Case D. Then electrolyze for the time calculated as in Case B.

Case F: Add 10 M gallium halide and concentrated halogen acid as calculated in Cases C and D.

Cases G, H, and I: Electrolyze at 75 amps. until there is a noticeable decrease in hydrogen being liberated at the cathode. Then reanalyze and redetermine the applicable case and act accordingly.

This invention has been described in terms of specific embodiments. However, it should be understood that this has been done for illustrative purposes only and therefore is not meant to limit the invention thereto. Many incidental changes may be incorporated into this process without departing from the spirit of this invention.

What is claimed is:

1. A process for preparing gallium by continuously electrolyzing an aqueous acidic solution of gallium ions comprising adding to said solution during the electrolysis sufficient quantities of gallium ions and halide anions to maintain the mole ratio of [anion]/ [Ga+ between about 2.5 to 1 and about 3.5 to 1 during the operation of the process.

2. A process according to claim 1 wherein said mole ratio is between about 2.9 to 1 and about 3.1 to -1.

3. A process according to claim 1 wherein said halide anion is a chloride ion.

4. A process according to claim 1 wherein said halide anion is a chloride ion and said mole ratio is between about 2.9 to 1 and about 3.1 to 1.

5. A process for preparing gallium by continuously electrolyzing an aqueous acidic solution of gallium ions comprising adding to said solution during the electrolysis suflicient quantities of gallium ions and halide anions, and withdrawing suflicient water and evolved gases from the electrolysis vessel during electrolysis, to maintain the mole ratio of [anion]/[Ga+ between about 2.5 to 1 and about 3.5 to 1 during the operation of the process.

6. A process according to claim 5 wherein said mole ratio is between about 2.9 to 1 and about 3.1 to 1.

8 "7. A process according to claim 5 wherein said halide anion is a chloride ion.

8. A process according to claim 5 wherein said halide anion is a chloride ion and said mole ratio is between about 2.9 to 1 and about 3.1 to 1.

OTHER REFERENCES Dopouidi Akad, Nauk. Ukr. R. S. R. 1955, No. 5, 462-463 (Chem. abstracts, vol. 50, p. 12693 g.)

15 HOWARD S. WILLIAMS, Primary Examiner.

H. M. FLOURNOY, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,423 ,301 January 21, 1969 Robert I. Stearns It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading to the printed specification, line 6, "245,189" should read 245,199 Column 6, line 8, ",BaBr should read GaBr line 25,

"Anion1" should read [Anion] Signed and sealed this 31st day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

1. A PROCESS FOR PREPARING GALLIUM BY CONTINUOUSLY ELECTROLYZING AN AQUEOUS ACIDIC SOLUTION OF GALLIUM IONS COMPRISING ADDING TO SAID SOLUTION DURING THE ELECTROLYSIS SUFFIECIENT QUANTITIES OF GALLIUM IONS AND HALIDE ANIONS TO MAINTAIN THE MOLE RATION OF (ANION)/(GA+3) BETWEEN ABOUT 2.5 RO 1 AND ABOUT 3.5 TO 1 DURING THE OPERATION OF THE PROCESS. 